1. Field of the Invention
The present invention relates to electron beam devices that utilize multi-staged depressed collectors for efficient collection of spent electrons. More particularly, the invention relates to an oil cooling system for a multi-staged depressed collector that provides good heat dissipation and high voltage standoff between adjacent collector stages.
2. Description of Related Art
It is known in the art to utilize a linear beam device, such as a klystron or travelling wave tube (TWT), for amplification of microwave signals in microwave systems. Such devices generally include an electron emissive cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture. One class of linear beam device, referred to as an inductive output amplifier, or inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode. The density modulated beam is accelerated by the anode, and propagates across a gap provided downstream within the inductive output amplifier. RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the cavity in the form of a high power, modulated RF signal.
At the end of its travel through the linear beam device, the electron beam is deposited into a collector or beam dump that effectively captures the remaining energy of the spent electron beam. The electrons that exit the drift tube of the linear beam device are captured by the collector and returned to the cathode voltage source. Much of the remaining energy in the electrons is released in the form of heat when the particles strike a stationary element, such as the walls of the collector. This heat loss constitutes an inefficiency of the linear beam device, and as a result, various methods of improving this efficiency have been proposed.
One such method is to operate the collector at a xe2x80x9cdepressedxe2x80x9d potential relative to the body of the linear beam device. In a typical linear beam device, the body of the linear beam device is at ground potential and the cathode potential is negative with respect to the body. The collector voltage is xe2x80x9cdepressedxe2x80x9d by applying a potential that is between the cathode potential and ground. By operating the collector at a depressed state, the negative electric field within the collector slows the moving electrons so that the electrons can be collected at reduced velocities. This method increases the electrical efficiency of the RF device as well as reducing undesirable heat generation within the collector.
It is also common for the depressed collector to be provided with a plurality of electrodes arranged in sequential stages, a structure referred to as a multi-staged depressed collector. Electrons exiting the drift tube of the linear beam device actually have varying velocities, and as a result, the electrons have varying energy levels. To accommodate the differing electron energy levels, the respective electrode stages have incrementally increasing negative potentials applied thereto with respect to the linear device body, such that an electrode having the highest negative potential is disposed the farthest distance from the interaction structure. This way, electrons having the highest relative energy level will travel the farthest distance into the collector before being collected on a final one of the depressed electrodes. Conversely, electrons having the lowest relative energy level will be collected on a first one of the depressed electrodes. By providing a plurality of electrodes of different potential levels, each electron can be collected on a corresponding electrode that most closely approximates the electron""s particular energy level. Thus, efficient collection of the electrons can be achieved. The significant efficiency improvement achieved by using a multi-staged depressed collector with an inductive output tube is described in U.S. Pat. No. 5,650,751, which is specifically incorporated by reference herein.
There are two significant drawbacks of multi-staged depressed collectors that must be controlled in order to have satisfactory operation. First, multi-staged depressed collectors generate a great deal of heat due to the electrons that impact the collector electrodes, and this heat must be dissipated to maintain an efficient level of operation and to prevent damage to the collector structure. Second, the adjacent electrode stages must be insulated from one another to prevent arcing due to the high voltages applied to the electrode stages. The known methods for controlling these problems often results in increasing the size and weight of the collector, so that it often becomes larger and heavier than the rest of the linear beam device.
More particularly, multi-staged depressed collectors are generally cooled using water or air as a cooling medium. To enable heat dissipation, a cooling surface is provided on an external portion of the collector that is in contact with the cooling medium. The cooling surface may be relatively small if water is used as a cooling medium, but needs to be relatively large if air is used. Since water contains positive and negative ions, high voltage electric fields tend to induce an ion current within the water. Therefore, in a water-cooled multi-staged depressed collector, the high voltages between the collector stages make it necessary to use very clean, deionized water in the water-cooling system and substantial lengths of insulating hoses to conduct the a cooling water between the individual electrode stages and between the electrode stages and ground in order to keep the ion current below a certain limit. The hoses further include seals that are susceptible to water leakage. Moreover, the water must be filtered and its resistance periodically checked; otherwise, the cooling surfaces may experience severe damage due to corrosion. An additional problem with water-cooled systems is that the hoses take up a lot of space, which defeats the advantage of having a relatively small cooling surface. Yet another problem with water-cooled systems is that the hoses cause a pressure drop in the cooling system that results in a reduction of the flow rate through the system. Lastly, unless glycol is mixed with the water, a water-cooled system will freeze at temperatures below 0xc2x0 C., which is unacceptable for certain applications.
While corrosion is not an issue with air-cooled systems, such systems have other disadvantages. Particularly, air-cooled multi-staged depressed collectors need large cooling fins because of the relatively poor thermal conductivity and specific heat of air. As a result, the dissipated power of an air-cooled multi-staged depressed collector is limited to about 40 KW because it is impractical to provide a sufficiently large cooling surface to keep the temperature within an acceptable range at higher power levels. Also, an air-cooled system requires large diameter ducts and therefore a lot of space. Dust must be filtered from the air-cooled system, and the filters result in pressure drops that reduce the volume of air flow. Since the cooling surface of the collector is larger with an air-cooled system than with a water-cooled system, the metallic parts of the collector experience a greater amount of thermal expansion and oxidation of the exposed metal surfaces. Each of these factors increases the stress on the collector, which degrades the useful life of the electron beam device. A final disadvantage of air-cooling systems is that they tend to be noisy, which makes the work environment undesirable.
Generally, multi-staged depressed collectors include insulating ceramic elements provided between the adjacent electrode stages to prevent arcing in air at maximum voltage. The space between the electrode stages must be large enough to hold off a high voltage within an extreme operating environment, such as at 8,000 feet above sea level, or in high humidity, or while exposed to a certain amount of dust. The hoses used in water-cooled systems that extend between stages further exacerbate the difficulty of controlling arcing by deforming the electric fields.
Accordingly, it would be very desirable to provide a cooling system for a multi-staged depressed collector that overcomes these significant drawbacks with conventional air and water-cooled systems. Such a cooling system would ideally achieve good heat dissipation and high voltage standoff between adjacent collector stages, without increasing the overall size of the collector.
In accordance with the teachings of the present invention an oil-cooling system is provided for a multi-staged depressed collector of a linear beam device, such as an inductive output tube or klystron. As known in the art, a multi-staged depressed collector comprises a plurality of electrode stages adapted to have respective electric potentials applied thereto. The electrode stages being separated from one another by respective electrical insulators. The oil-cooling system of the present invention provides cooling to the entire surface of the collector, including the electrode stages and the electrical insulators. Oil resists voltage breakdown, and permits a cooling structure that takes up less space than air or water-cooling systems.
More particularly, the electrode stages are provided with a plurality of channels that extend along the outer surfaces of the electrodes. In an embodiment of the invention, an inner sleeve is disposed in contact with the outer surface of the electrode stages and substantially encloses the plurality of channels. An outer sleeve encloses the inner sleeve with a space defined therebetween. The inner sleeve further includes an opening at an end thereof providing an oil communication path between the space between the inner and outer sleeves, and the plurality of channels. An oil source is coupled to one of the inner sleeve and the outer sleeve in order to provide a flow of oil therethrough. The channels may extend axially along the outer surface of the electrodes, or alternatively, helical channels may be provided.
A more complete understanding of the oil-cooled multi-staged depressed collector will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.